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Various Neural
Networks
Neural Networks

A mathematical model to solve engineering problems


Tasks




Group of connected neurons to realize compositions of non
linear functions
Classification
Discrimination
Estimation
2 types of networks


Feed forward Neural Networks
Recurrent Neural Networks
Feed Forward Neural Networks

Output layer

2nd hidden
layer
1st hidden
layer
x1
x2

…..
xn
The information is
propagated from the
inputs to the outputs
Computations of
functions from n input
variables by compositions
of algebraic functions
Time has no role (NO
cycle between outputs
and inputs)
Recurrent Neural Networks


0
1

0

0
1

0
0
1
x1
Can have arbitrary topologies
Can model systems with
internal states (dynamic ones)
Delays are associated to a
specific weight
Training is more difficult
Performance may be
problematic

x2
Stable Outputs may be more
difficult to evaluate
 Unexpected behavior
(oscillation, chaos, …)
Properties of Neural Networks


Supervised networks are universal approximators
networks)
Theorem : Any limited function can be approximated by
a neural network with a finite number of hidden neurons
to an arbitrary precision
Supervised learning
The desired response of the neural
network in function of particular inputs is
well known.
 A “Professor” may provide examples and
teach the neural network how to fulfill a
certain task

Unsupervised learning



Idea : group typical input data in function of
resemblance criteria un-known a priori
Data clustering
No need of a professor

The network finds itself the correlations between the
data
 Examples of such networks :

Kohonen feature maps
Classification (Discrimination)
Class objects in defined categories
 Rough decision OR
 Estimation of the probability for a certain
object to belong to a specific class
Example : Data mining
 Applications : Economy, speech and
patterns recognition, sociology, etc.

Example
Examples of handwritten postal codes
drawn from a database available from the US Postal service
What needed to create NN ?


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Determination of relevant inputs
Collection of data for the learning and testing
phases of the neural network
Finding the optimum number of hidden nodes
Learning the parameters
Evaluate the performances of the network
If performances are not satisfactory then review
all the precedent points
Popular neural architectures
Perceptron
 Multi-Layer Perceptron (MLP)
 Radial Basis Function Network (RBFN)
 Time Delay Neural Network (TDNN)
 Other architectures

Perceptron




Rosenblatt (1962)
Linear separation
Inputs :Vector of real values
Outputs :1 or -1
y  step0(v)
c0
1
+
++
+ +
+
+
+ + + ++ +
+
+ +
+
+
+
++ +
+ + + + ++
+ +
+
+
+
+
y0
++
 v  c0  c1 x1  c2 x2
c1
c2
x1
x2
y 1
c0  c1 x1  c2 x2  0

The perceptron algorithm converges if
examples are linearly separable
Multi-Layer Perceptron

Output layer
2nd hidden
layer
1st hidden
layer
Input data
One or more hidden
layers
Different non linearly separable
problems
Structure
Single-Layer
Two-Layer
Three-Layer
Types of
Decision Regions
Exclusive-OR
Problem
Half Plane
Bounded By
Hyperplane
A
B
B
A
Convex Open
Or
Closed Regions
A
B
Abitrary
(Complexity
Limited by No.
of Nodes)
B
A
A
B
B
A
Classes with Most General
Meshed regions Region Shapes
B
B
B
A
A
A
Radial Basis Functions

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A radial basis function (RBF) is a real-valued
function whose value depends only on the
distance from some other point c, called a
center, φ(x) = f(||x-c||)
Any function φ that satisfies the property φ(x) =
f(||x-c||) is a radial function.
The distance is usually the Euclidean distance

|| x  c ||  i 1 xi  ci
2
N

2
Radial Basis Functions


The popular output of radial basis functions is
the Gaussian function:
 x  cj

 x  cj
 exp( a
 j

2

 )


a=1, c1=0.75, c2=3.25
Radial Basis Functions Network
(RBFN)

Features

One hidden layer

The activation of a hidden unit is determined by a radial basis function
Outputs
Radial units
Inputs


Generally, the hidden unit function is the
Gaussian function
The output Layer is linear:

s( x)   j 1W j  x  c j
K

 x  cj


 x  cj
 exp( wj
 j

2

 )


RBFN Learning

The training is performed by deciding on
 How
many hidden nodes there should be
 The centers and the sharpness of the Gaussians

2 steps
 In
the 1st stage, the input data set is used to
determine the parameters of the RBF
 In the 2nd stage, RBFs are kept fixed while the
second layer weights are learned ( Simple BP
algorithm like for MLPs)
Time Delay Neural Network (TDNN)


Introduced by Waibel in 1989
Properties
 Local,
shift invariant feature extraction
 Notion of receptive fields combining local information
into more abstract patterns at a higher level
 Weight sharing concept (All neurons in a feature
share the same weights)


All neurons detect the same feature but in different position
Principal Applications
 Speech
recognition
 Image analysis
TDNNs (cont’d)
Hidden
Layer 2


Hidden
Layer 1

Inputs
Objects recognition in an
image
Each hidden unit receive
inputs only from a small
region of the input space :
receptive field
Shared weights for all
receptive fields =>
translation invariance in
the response of the
network

Advantages
 Reduced
number of weights
Require fewer examples in the training set
 Faster learning

 Invariance
under time or space translation
 Faster execution of the net (in comparison of
full connected MLP)
Summary

Neural networks are utilized as statistical tools

Adjust non linear functions to fulfill a task
 Need of multiple and representative examples but fewer than in other
methods

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Neural networks enable to model complex static phenomena (FeedForward) as well as dynamic ones (Recurent NN)
NN are good classifiers BUT


Good representations of data have to be formulated
Training vectors must be statistically representative of the entire input
space
 Unsupervised techniques can help

The use of NN needs a good comprehension of the problem